Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO or SnO:F) coatings are widely used as window electrodes in opto-electronic devices. These transparent conductive oxides (TCOs) have been immensely successful in a variety of applications. Unfortunately, however, the use of ITO and FTO is becoming increasingly problematic for a number of reasons. Such problems include, for example, the fact that there is a limited amount of the element indium available on Earth, the instability of the TCOs in the presence of an acide or base, their susceptibility to ion diffusion from ion conducting layers, their limited transparency in the near infrared region (e.g., power-rich spectrum), high leakage current of FTO devices caused by FTO structure defects, etc. The brittle nature of ITO and its high deposition temperature can also limit its applications. In addition, surface asperities in SnO2:F may cause problematic arcing.
Thus, it will be appreciated that there is a need in the art for smooth and patternable electrode materials with good stability, high transparency, and excellent conductivity.
The search for novel electrode materials with good stability, high transparency, and excellent conductivity is ongoing. One aspect of this search involves identifying viable alternatives to such conventional TCOs. In this regard, the inventor of the instant invention has developed a viable transparent conductive coating (TCC) based on carbon, specifically graphene.
The term graphene generally refers to one or more atomic layers of graphite, e.g., with a single graphene layer or SGL being extendible up to n-layers of graphite (e.g., where n can be as high as about 10). Graphene's recent discovery and isolation (by cleaving crystalline graphite) at the University of Manchester comes at a time when the trend in electronics is to reduce the dimensions of the circuit elements to the nanometer scale. In this respect, graphene has unexpectedly led to a new world of unique opto-electronic properties, not encountered in standard electronic materials. This emerges from the linear dispersion relation (E vs. k), which gives rise to charge carriers in graphene having a zero rest mass and behaving like relativistic particles. The relativistic-like behavior delocalized electrons moving around carbon atoms results from their interaction with the periodic potential of graphene's honeycomb lattice gives rise to new quasi-particles that at low energies (E<1.2 eV) are accurately described by the (2+1)-dimensional Dirac equation with an effective speed of light νF≈c/300=106 ms−1. Therefore, the well established techniques of quantum electrodynamics (QED) (which deals with photons) can be brought to bear in the study of graphene—with the further advantageous aspect being that such effects are amplified in graphene by a factor of 300. For example, the universal coupling constant α is nearly 2 in graphene compared to 1/137 in vacuum. See K. S. Novoselov, “Electrical Field Effect in Atomically Thin Carbon Films,” Science, vol. 306, pp. 666-69 (2004), the contents of which are hereby incorporated herein.
Despite being only one-atom thick (at a minimum), graphene is chemically and thermally stable (although graphene may be surface-oxidized at 300 degrees C.), thereby allowing successfully fabricated graphene-based devices to withstand ambient conditions. High quality graphene sheets were first made by micro-mechanical cleavage of bulk graphite. The same technique is being fine-tuned to currently provide high-quality graphene crystallites up to 100 μm2 in size. This size is sufficient for most research purposes in micro-electronics. Consequently, most techniques developed so far, mainly at universities, have focused more on the microscopic sample, and device preparation and characterization rather than scaling up.
Unlike most of the current research trends, to realize the full potential of graphene as a possible TCC, large-area deposition of high quality material on substrates (e.g., glass or plastic substrates) is essential. To date, most large-scale graphene production processes rely on exfoliation of bulk graphite using wet-based chemicals and starts with highly ordered pyrolytic graphite (HOPG) and chemical exfoliation. As is known, HOPG is a highly ordered form of pyrolytic graphite with an angular spread of the c axes of less than 1 degree, and usually is produced by stress annealing at 3300 K. HOPG behaves much like a pure metal in that it is generally reflective and electrically conductive, although brittle and flaky. Graphene produced in this manner is filtered and then adhered to a surface. However, there are drawbacks with the exfoliation process. For example, exfoliated graphene tends to fold and become crumpled, exists as small strips and relies on a collage/stitch process for deposition, lacks inherent control on the number of graphene layers, etc. The material so produced is often contaminated by intercalates and, as such, has low grade electronic properties.
An in-depth analysis of the carbon phase diagram shows process window conditions suitable to produce not only graphite and diamond, but also other allotropic forms such as, for example, carbon nano-tubes (CNT). Catalytic deposition of nano-tubes is done from a gas phase at temperatures as high as 1000 degrees C. by a variety of groups.
In contrast with these conventional research areas and conventional techniques, certain example embodiments of this invention relate to a scalable technique to hetero-epitaxially grow mono-crystalline graphite (n as large as about 15) and convert it to high electronic grade (HEG) graphene (n<about 3). Certain example embodiments also relate to the use of HEG graphene in transparent (in terms of both visible and infrared spectra), conductive ultra-thin graphene films, e.g., as an alternative to the ubiquitously employed metal oxides window electrodes for a variety of applications (including, for example, solid-state solar cells). The growth technique of certain example embodiments is based on a catalytically driven hetero-epitaxial CVD process which takes place a temperature that is low enough to be glass-friendly. For example, thermodynamic as well as kinetics principles allow HEG graphene films to be crystallized from the gas phase on a seed catalyst layer at a temperature less than about 700 degrees C.
Certain example embodiments also use atomic hydrogen, which has been proven to be a potent radical for scavenging amorphous carbonaceous contamination on substrates and being able to do so at low process temperatures. It is also extremely good at removing oxides and other overlayers typically left by etching procedures.
Certain example embodiments relate to a solar cell. The solar cell comprises a glass substrate. A first graphene-based conductive layer is located, directly or indirectly, on the glass substrate. A first semiconductor layer is in contact with the first graphene-based conductive layer. At least one absorbing layer is located, directly or indirectly, on the first semiconductor layer. A second semiconductor layer is located, directly or indirectly, on the at least one absorbing layer. A second graphene-based conductive layer is in contact with the second semiconductor layer. A back contact is located, directly or indirectly, on the second-graphene-based conductive layer.
In certain example embodiments, the first semiconductor layer is an n-type semiconductor layer and the first graphene-based layer is doped with n-type dopants, and the second semiconductor layer is a p-type semiconductor layer and the second graphene-based layer is doped with p-type dopants. In certain example embodiments, a layer of zinc-doped tin oxide is interposed between the glass substrate and the first graphene-based layer. The first and/or second semiconductor layers may comprise polymeric material(s) in certain example embodiments.
Certain example embodiments relate to a photovoltaic device. The photovoltaic device comprises a substrate; at least one photovoltaic thin-film layer; first and second electrodes; and first and second transparent, conductive graphene-based layers. The first and second graphene-based layers are respectively doped with n- and p-type dopants.
Certain example embodiments relate to a touch panel subassembly. The touch panel subassembly comprises a glass substrate. A first transparent, conductive graphene-based layer is provided, directly or indirectly, on the glass substrate. A deformable foil is provided, with the deformable foil being substantially parallel and in spaced apart relation to the glass substrate. A second transparent, conductive graphene-based layer is provided, directly or indirectly, on the deformable foil.
In certain example embodiments, the first and/or second graphene-based layer(s) is patterned. A plurality of pillars may be located between the deformable foil and the glass substrate, and at least one edge seal may be provided at the periphery of the subassembly in certain example embodiments.
Certain example embodiments relate to a touch panel apparatus comprising such a touch panel subassembly. A display may be connected to a surface of the substrate of the touch panel subassembly opposite the deformable foil. The touch panel apparatus may be a capacitive or resistive touch panel apparatus in certain example embodiments.
Certain example embodiments relate to a data/bus line, comprising a graphene-based layer supported by a substrate. A portion of the graphene-based layer has been exposed to an ion beam/plasma treatment and/or etched with H*, thereby reducing conductivity of the portion. In certain example embodiments, the portion is not electrically conductive. In certain example embodiments, the substrate is a glass substrate, silicon wafer, or other substrate. In certain example embodiments, the portion may be at least partially removed by exposure to the ion beam/plasma treatment and/or the etching with H*.
Certain example embodiments relate to an antenna. A graphene-based layer is supported by a substrate. A portion of the graphene-based layer has been exposed to an ion beam/plasma treatment and/or etched with H* to thin the portion of graphene-based layer in comparison to other portions of the graphene-based layer. The graphene-based layer, as a whole, has a visible transmission of at least 80%, more preferably at least 90%.
Certain example embodiments relate to a method of making an electronic device. A substrate is provided. A graphene-based layer is formed on the substrate. The graphene-based layer is selectively patterned by one of: ion beam/plasma exposure and etching with H*.
In certain example embodiments, the graphene-based layer is transferred to a second substrate prior to the patterning. In certain example embodiments, the patterning is performed to reduce conductivity and/or remove portions of the graphene-based layer.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.